In many processing and computing systems, magnetic data storage devices such as disk drives are used for storing data. A typical disk drive includes a spindle motor for rotating one or more data storage disks having data storage surfaces, a head arm that supports one or more transducer heads, and an actuator for moving the heads radially across the disks to enable the heads to write data to and read data from concentric tracks on the disks.
In general, the head is positioned very close to the corresponding disk surface. Typical clearance between the head and a smooth disk surface is about one microinch or less. The close proximity of the head to the disk surface allows recording very high resolution data and servo patterns on the disk surface. Servo patterns are typically written into servo sectors with uniform angular spacing of the servo sectors and data sectors or blocks interleaved between the servo sectors. An example servo pattern includes circumferentially sequential, radially staggered single frequency bursts. Servo patterns provide the disk drive with head position information to enable the actuator to move the head from starting tracks to destination tracks during random access track seeking operations. Further, the servo patterns provide the disk drive with head position information to enable the actuator to position and maintain the head in proper alignment with a track during track following operations when user data is written to or read from the available data sectors in concentric tracks on the disk surface.
Heads currently use dual elements. An inductive write element having a relatively wide recording gap writes information in the tracks, and a read element such as a giant magneto-resistive (GMR) sensor having a relatively narrow playback gap reads information from the tracks. With this arrangement, track densities equaling and exceeding 30,000 tracks per inch (TPI) are possible.
In a standard manufacturing process, a head-disk assembly (HDA) of the disk drive is assembled in a clean room and then transported to a specialized servo writer where the HDA is mounted on a stabilized metrological measurement system. The drive modules are then assembled to the HDA and the drive is moved to a self-scan station where the drive is tested for reliable servo operation. Block error information, drive defect information, drive specific control track information and other information is written to the drive at this station. If the drive fails the self-scan tests, it is either reworked or scrapped at this late manufacturing stage.
Conventionally, servo patterns are written into the servo sectors of each disk using a servo writer in the drive assembly process before the HDA is sealed against particulate contamination. The servo writer is a complex and expensive machine, typically stabilized on a large granite base to minimize unwanted vibration, and employs laser interferometry for precise position measurements. The servo writer typically requires direct mechanical access to the head arm and includes a fixed head for writing a clock track onto a disk surface.
Because of the need for direct access to the interior of the HDA, servo writers are typically located within a clean room where air is purged of impurities that might otherwise interfere with the servo writing or normal drive operation after manufacturing. Servo writers occupy a large portion of the clean room factory floor. Further, servo-writing by the servo writer is very time consuming. In one example, a disk drive having two disks with four disk surfaces can require three servo writer controlled passes of the head over a single track, consuming a total servo writing time as long as 13.2 minutes. Thus, servo writing using servo writers in clean rooms requires both considerable is capital investment and severe time penalties attributable to servo writer bottlenecks. Further, as track densities increase with evolving disk drive designs, servo writers have to be replaced or upgraded at considerable capital expense.
An attempt to alleviate the above shortcomings is directed to servo writing a reference pattern at full resolution on one surface of a reference disk during a pre-assembly operation. The reference disk with the reference pattern is assembled with blank disks into an HDA. After the disk drive is sealed, the disk drive uses the reference pattern to self-write embedded servo patterns on each disk surface within the disk drive. Finally, the reference pattern is erased, leaving the disk drive with properly located servo patterns on every disk surface, including the disk surface which originally included the reference pattern. An example of self-servo writing is described in U.S. Pat. No. 5,012,363 to Mine et al. entitled “Servo Pattern Writing Method For A Disk Storage Device”. However, a disadvantage of this approach is that certain repeatable runout (RRO) information must be removed during the self-servo write operation. Another disadvantage is that a servo writer is required to write the reference pattern on the reference disk.
A self-servo writing method which eliminates the need for servo writers is described in commonly assigned U.S. Pat. No. 5,668,679 to Swearingen et al. entitled “System For Self-Servo Writing A Disk Drive”, the disclosure thereof being incorporated herein by reference. This method includes writing a clock track at an outer diameter (OD) recording region of a disk surface of a disk drive, tuning an open-loop seek from the OD to an inner diameter (ID) recording region of the disk surface to develop a repeatable seek profile, and recording high frequency spirals from the OD to the ID with each spiral including embedded (e.g. missing bit) timing information. Then spiral provided peak data and missing bit data are read back. A voltage controlled oscillator (VCO) is locked to the timing information to track disk angular position. As the head is moved radially from the OD to the ID, the detected spiral peaks shift in time relative to a starting (index) mark although the timing information does not shift. Servo patterns can then be precisely written across the disk surface by multiplexing between reading the spirals and writing the servo patterns. After the integrity of the servo patterns has been verified, the spirals are erased (overwritten with user data). While this method is satisfactory, challenges remain in generating and recording an accurate clock track on the disk surface. Further, the time required to produce the spirals on the disk surface can be lengthy.
Another approach involves the use of a low resolution (low density) reference pattern (bursts) transferred to a reference disk by magnetic printing, and self-servo writing high resolution final servo patterns using the reference pattern. The reference disk with the magnetically printed reference pattern is known as printed media. However, printed media signal-to-noise ratio (SNR) is marginal for existing disk drives due to the low pattern density available with current printing techniques. One conventional approach to improving the SNR of printed media involves reducing the feature size in the printed media to increase the reference pattern density, but this is not practical due to limitations in optics and lithography. Another approach is increasing the length of the reference pattern bursts (every doubling of burst length improves SNR by 3 dB) but the disk real estate available for more than one doubling is expensive. In another approach, increasing the angle of the bursts leads to smaller feature sizes but this also leads to printing difficulties as the angle increases. Yet another approach involves zoned printing by changing the reference pattern density at the disk ID and OD zones to improve the SNR but this leads to difficulties patching the reference patterns between the zones (particularly in the presence of eccentricity).
For printed media reference patterns, the limitations in spatial resolution of the printing process make it impractical to space transitions as closely as the servo patterns that are self-written with the disk drive heads. When existing digital demodulation techniques are used on the printed reference pattern, the resulting signal has significantly lower SNR than the servo patterns. There is a need for improving the SNR of printed media reference patterns used for disk drive self-servo writing.